HYBRID NOTCH FILTER

Abstract: A hybrid notch filter comprises an impedance inverter network connected across a two port filter network. In its simplest form, the impedance inverter network comprises impedance inverters of substantially 2ROOT 2 characteristic admittance connected between the input and output ports across which the two port filter network is connected, and impedance inverters of substantially unity characteristic admittance interconnecting the 2ROOT 2 characteristic admittance impedance inverters at their respective ends. The two port filter network may be a plurality of serially connected resonators and is connected across the output ports of the impedance inverter network. The hybrid notch filter may be formed into a switched hybrid notch filter by providing a switch between an adjacent pair of resonators in the filter network.

Description: HYBRID NOTCH FILTER This invention relates to a microwave filter and more particularly to a hybrid notch microwave filter. Narrow bandwidth bandstop filters (or notch filters) are frequently required in microwave communications systems. It is often important for such systems to be able to switch into an "all-pass" state with a minimum level of loss and time delay distortion. A rejection level of 20 to 30dB is typically required. Our United Kingdom patent application No. 9315644.6 discloses a switched bandstop filter arrangement which comprises a bandstop filter arrangement having a bandpass filter operatively connectable in parallel therewith via switching means incorporated in the bandpass filter. The arrangement exhibits a number a desirable properties, including the minimisation of loss in the "all-pass" state outside the bandstop region and the minimisation of dynamic range limitations due to the switches. However, the "all-pass" network is of the same degrees as the bandstop characteristic, with the result that significant distortion occurs of short pulses at frequencies within the bandstop band, when the filter arrangement is switched into the "all-pass" state. We have now devised a filter which overcomes the problems outlined above, which in one form comprises a notch filter and in another form comprises a switched notch filter. In accordance with this invention, there is provided a hybrid notch filter which comprises an impedance inverter network connected across a two port filter network. In a preferred embodiment, the impedance inverter network comprises impedance inverters of substantially V2 characteristic admittance connecting the input and output ports with respective ones of two nodes across which the two port filter network or connected, and respective impedance inverters of substantially unity characteristic admittance interconnecting those two nodes and also interconnecting the input and output ports of the impedance inverter network. Preferably the two port filter network comprises a plurality of serially connected resonators, typically a Chebyshev filter. The two nodes of the impedance inverter network, across which the two port network is connected, may be incorporated in the first and last resonators of the two port filter network. The hybrid notch filter may be formed into a switched hybrid notch filter by providing a switch between an adjacent pair of resonators in the filter network. Embodiments of this invention will now be described by way of examples only and with reference to the accompanying drawings, in which: FIGURE 1 is an impedance inverter representation of a 3dB hybrid, for use in explaining this invention; FIGURE 2 is a diagram of a hybrid notch filter in accordance with this invention; FIGURE 3 is a diagram of a 6th degree Chebyshev-type filter; FIGURE 4 is a diagram of a 6th degree hybrid notch filter in accordance with this invention; FIGURE 5 is a diagram showing the theoretical performance characteristics of the filter of Figure 4; and FIGURE 6 is a diagram showing the measured performance characteristics of an experimental filter built in accordance with Figure 4. Referring firstly to Figure 1 of the drawings for the purpose of explanation, an ideal 3dB directional coupler or hybrid is shown, represented by a network of four ideal impedance inverters, two having a characteristic admittance of V2 (connecting ports 1,2 and 3,4 respectively) and two having a characteristic impedance of unity (connecting ports 1,3 and 2,4 respectively) . The even mode network for this arrangement has an even mode admittance: and an odd mode admittance: yielding a reflection coefficient: P e =0 and a transmission coefficient: T. = 111 Similarly, for the odd mode network: Thus for an input at port (1) , the reflection at port (1) is: P +P e °=Q whilst the output at port (3) is: P -P e ° =o and the output at port (2) is: 2 " J2 and the output at port (4) is: T - T hence providing hybrid properties. Consider a symmetrical two port network defined by its even and odd mode admittances Ye and Yo. Its corresponding scattering parameters are therefore: ώ ll~ α+ig (i+y 0 ) S '1.2,=- ι γ .- y 0 (i+y β ) (i+y 0 ) Figure 2 shows such a two port network connected across the output ports of the hybrid of Figure 1. The even mode admittance for the new two-port network formed between ports (1) and (3) of the hybrid is: and the corresponding odd mode admittance is: and the associated reflection coefficient is: and the associated transmission coefficient is: Hence in Figure 2 a new network has been created in which the reflection and transmission characteristics have been interchanged apart from a ±90° phase shift. For example, if the original two port network was a Chebyshev filter, e.g. as shown in Figure 3 , then the new network is an Inverse Chebyshev filter. In many technologies, narrowband bandpass filters can be readily constructed but direct bandstop devices may be difficult due to the electric spacing of adjacent resonators. A hybrid notch filter as shown in Figure 2 overcomes this problem, although difficulties in producing exact values for the impedance inverters will limit levels of attenuation, typically to no more than about 30dB. If the bandpass filter is realised from a network containing shunt capacitors and impedance inverters, then the addition of the hybrid of Figure 1 as the impedance inverter network produces the appropriate bandstop filter. Scaling for a narrow bandwidth notch filter enables 3 of the inverters of the hybrid to be absorbed into the filter as two input couplings and as an additional coupling between the first and last resonators, producing an overall filter similar to a bandpass filter with input and output connected to a straight through line of unity normalised impedance and 90° long electrically. Such an arrangement is shown in Figure 4. Suppose that the two-port bandpass network is a ladder structure constructed with a single switch connected at the centre of the filter. In the closed state the network is unchanged, but in the open state both the even mode and odd mode admittance become the same as the even mode admittance Ye. In this state the overall even mode admittance is: + y e + y e but the odd mode is: -i → +2 1 - j Ye - + y e - + y e resulting in a network where: -.11 '11 = 0 which is an all-pass network when the original network is lossless. In this form, the switched notch filter has very similar properties to the filter arrangement disclosed in our United Kingdom patent application No. 9315644.6, but only requires a single switch. However, simple shunt switches may be incorporated into all the resonators apart from the first and last each of which has admittance C,p. If this network is now switched into the 'all-pass' state: s Ao in_ -J ' CL -qp) _5 12 - ■ (1+ j p) which is a simple first degree section providing a minimal level of distortion to pulses whilst retaining the desirable features of low loss out of band and high dynamic range due to the decoupling of the switches. If this type of switching is used, then the bandpass filter may be designed with additional cross-coupling between resonators to provide an elliptic function response. This then gives an optimum response in the bandstop case whilst providing the single degree "all-pass" state when switched. An alternate form of hybrid notch filter may be formed as follows. Consider the network shown in Figure 2 where the two-port network is no longer symmetrical and defined by the scattering parameters S u , S 22 and s ]2 where the input is connected to port 2 of the hybrid. An input signal at port 3 will result in input signals to the two-port of > 2 from port 2 and '^ 2 from port 4. The corresponding reflected signals will then produce outputs at port 1 of: J < ιι + S 22 ) and at port 3 of: ^ 22 S l ) However in addition to the signals, the signals transmitted through the two-port will produce outputs at port 1 of: 1 ( -S 12 + S 12 ) =Q and at port 3 of: ^ ( S 12 +S 12 ) Hence, the overall network has the scattering parameters: O /- 1 ^22 ~ | 5'll +js 12 11 2 e /_ J n °22 i ε 12 ,_ j ( s 11 + s 22 ) ώ 12 o In the case of a symmetrical two-port, S n = S 22 and we recover equations (14) and (15) . However, several other degenerate forms are possible. With S π = S 22 and S I2 = 0, we have the familiar all-pass form with equal reactances terminating ports 2 and 4 of the hybrid. Additionally, if S 12 = 0 and ports 2 and 4 of the hybrid are individually terminated with the even and odd mode admittances of the symmetrical two- port shown in Figure 2, then the overall two-port is still described by equations (14) and (15) apart from a constant phase shift. We have built and tested a 6th degree hybrid notch filter in accordance with Figure 4, to meet a severe specification at 850MHz encountered in cellular base stations. Located in the front end of the receiver prior to the pre- amplifier, the filter has to reject to a level of 20dB over a bandwidth of 1MHz with less than 1.5dB loss outside a bandwidth of 1.5MHz. Initially, the filter was designed as a Chebyshev filter with 20dB return loss and unloaded Q factors of greater than 20,000. This was achieved using dielectric resonators supporting the TE U1Λ mode and is shown in Figure 3, in which the circles 1 to 6 represent the resonators and the lines the impedance inverter couplings. In order to convert the Chebyshev filter to a hybrid notch filter, the inverter network of Figure 2 is connected to it, but part of this hybrid may be absorbed into the first and last resonators 1,6 as shown in Figure 4, nodes 0 and 7 forming the input and output ports. Thus, physically the filter may be constructed with a 50Ω quarter wavelength line between nodes 0 and 7 with decoupling from node 0 into resonator 1 and from node 7 into resonator 6. Over the bandwidth of the bandstop region, the additional coupling between resonators 1 and 6 provides the fourth arm of the hybrid. Away from the stopband, the device then degenerates into a broadband 3rd degree network consisting of a 50Ω line between nodes 0 and 7 and simple shunt short circuited quarter wavelength stubs at both nodes. This network has a bandpass characteristic of sufficient bandwidth with a maximally flat response around the operating frequency for most applications. The original Chebyshev response provided by the network shown in Figure 3 may be readily modified to provide finite transmission zeros. If a small amount of negative coupling is introduced between nodes 1 and 6, then a pair of real axis and a pair of imaginary axis transmission zeros are introduced, producing a quasi-elliptic, phase equalised filter with enhanced performance for return loss and transmission loss levels of approximately 20dB. Using this arrangement in the hybrid notch filter shown in Figure 4 does not change the structure but simply modifies the coupling between nodes 1 and 6. This arrangement was used in the filter which we built and tested and the theoretical and measured characteristics are shown in Figures 5 and 6 which show close agreement. It will be appreciated that the hybrid notch filter and switched hybrid notch filter which have been described are useful in several applications. The former is ideal for notching out unwanted signals close to and within an operating band such as encountered in the cellular telephone industry. The latter has important applications in receiver systems which can be overloaded by frequency hopping high power transmitters close to the receiver.

Claims: CLAIMS 1) A hybrid notch filter comprising an impedance inverter network connected across a two port filter network. 2) A hybrid notch filter according to claim 1, wherein the impedance inverter network comprises impedance inverters of substantially V2 characteristic admittance connecting the input and output ports with respective ones of two nodes across which the two port filter network is connected, and respective impedance inverters of substantially unity characteristic admittance interconnecting the two nodes and also interconnecting the input and output ports of the impedance inverter network. 3) A hybrid notch filter according to any preceding claim, wherein the two port filter network comprises a plurality of serially connected resonators. 4) A hybrid notch filler according to claim 3, wherein the two port filter network forms a Chebyshev filter. 5) A hybrid notch filter according to claim 3 or 4, wherein the two nodes of the impedance inverter network, across which the two port network is connected, are incorporated in the first and last resonators of the two port filter network.- 6) A hybrid notch filter according to any preceding claim, wherein the filter is switched. 7) A switched hybrid notch filter according to claim 6 when dependant on any of claims 3-5, comprising a switch between an adjacent pair of resonators in the filter network.

Inventors: RHODES JOHN DAVID, SLEIGH PHILIP DAVID, RHODES, JOHN, DAVID, SLEIGH, PHILIP, DAVID
Applicants: FILTRONIC LTD, RHODES JOHN DAVID, SLEIGH PHILIP DAVID, FILTRONIC COMTEK PLC, RHODES, JOHN, DAVID, SLEIGH, PHILIP, DAVID
Application year: 1994
Main classification: H01P1
IPC codes: H01P1